This invention relates generally to methods for preparing and delivering a self-curing cement formed as a polymeric reaction product after mixing a powder polymer component with a liquid monomer component.
More particularly, the invention relates to methods and apparatuses for preparing and delivering bone cement in an operating room environment.
In many orthopedic surgical procedures, it is necessary to employ a bonding material to set implants such as pins and artificial joints in bones. The cement employed for such surgical purpose is generally a polymeric material which is prepared by copolymerization of its components as needed. Because of the necessity for fairly quick setting material, the cement is almost universally prepared in the operating room environment and during the course of an operation. The preparation of the cement involves mixing the aforementioned components, evaluating the consistency of the mixture and injecting into the patient""s tissues.
Care must be applied to create an homogeneous cement by thorough mixing of the components, and avoiding air bubbles. Sterility must be maintained both within the mixture itself, but also about the instruments used throughout the entire process. Care must be exercised not to contaminate the hands of the surgeon and his assistants.
The processing time, i.e., the time at the surgeon""s disposal for carrying out all the required work to anchor a prosthesis in correct position in a bone cavity, from the beginning of the introduction of the cement into the bone cavity until the hardening of the cement that no longer permits any change in the position of the prosthesis, is relatively short. Consequently, the bone cement mixing and delivering apparatus must be operable in a minimum of time.
Motorized, table-top bone cement mixing machines such as the ones disclosed in U.S. Pat. Nos. 5,571,282 Earle, and 5,975,751 Earle, like any other electro-mechanical devices of this type, are very difficult to sterilize. The mere manipulation of their control switches and levers is likely to compromise the sterility of the operator""s gloved hands. Moreover, this type of device fulfills only the preparation phase of the process and not the delivery of the cement into the bone cavity.
The process of transferring the bone cement from a mixture to a delivery device can be time-consuming and potentially contaminating. It is desirable to minimize the transfer to avoid contamination and save time. The most desirable method is to mix the components of the bone cement and deliver the final product into a bone cavity under an isobaric and isovolumic environment that will not draw in gases and contaminants into the bone cement.
Hand-operated devices combining bone cement mixing and injecting mechanisms have been proposed such as the one disclosed in U.S. Pat. Nos. 5,100,241 Chan; and 5,558,745 Tanaka et al. In those instruments, the components of the bone cement, prior to mixing, are kept in separate cartridges made of materials such as plastic that are not very suitable for the highly reactive monomer. The mixing occurs in non-isobaric and non-isovolumic environments and oftentimes under negative pressure relative to ambient atmosphere. Impurities may be drawn into the cement and partial evaporation of the monomer may create weakening bubbles in the cement.
None of those prior art devices provides a convenient way to test the viscosity of the cement, the surgeon must resort to rubbing a small bead of the cement between thumb and forefinger in order to access its consistency and viscosity.
The instant invention results from attempts to develop a more effective and practical method and apparatus for quickly and safely mixing the components of a bone cement, assess the viscosity of the resulting mixture, and inject it in a timely manner into a bone cavity.
The principal and secondary object of this invention are to provide a device and method for conveniently preparing and delivering a polymerized bone cement under sterile, isobaric and isovolumic conditions in an operating room environment while safeguarding the integrity of the operator""s sterile apparel, avoiding the admission of impurities and the formation of weakening gas bubbles and providing a rapid and convenient way to assess the viscosity of the material prior to injection.
These and other valuable objects are achieved by packaging a volume of polymer powder in a sterilized and evacuated cylindrical vessel which contains a disk agitator mounted on a tubular axial shaft manipulable through an external handle, and a piston that can be selectively connected to the disk for translation by means of the same shaft and handle after the mixture has been thoroughly mixed. The vessel is packaged in a sterile and evacuated pliable envelope. The liquid monomer is drawn into a syringe then injected through the sterile envelope and an elastomeric, self-sealing diaphragm in the wall of the vessel. A tubular, distal portion of the shaft that passes through the outlet port can be broken off allowing the shaft to be withdrawn to open the outlet. The broken-off part can be mounted on a bushing surrounding the outlet to form a nozzle. The viscosity of the mixture can be assessed by letting the agitator and shaft move under their own weight through the mixture while the vessel is held in a vertical position.